West German Patent No. DE 2,742,115 discloses a shutoff and regulating valve of this type, the stem of which is sealed off by means of two sealing elements arranged in the housing cover. The cover has an annular groove on its underside into which the first sealing element is inserted. Arranged between the stem and the stopcock is an intermediate piece having a T-shaped cross section which engages into a square recess of the stopcock. This intermediate connecting piece, on its upper side facing the stem, has a larger diameter than the stem. The first sealing element rests on the upper side of the connecting piece, and the sealing effect of the first sealing element is dependent on the axial position of the connecting piece and/or of the stem. Since the axial position of the stem varies as a result of production tolerances, abrasion, external forces and the like, difficulties can arise with regard to safety regulations and environmental protection regulations, in ensuring that the requisite limiting values for leakage rates can be maintained even throughout a long lifetime. If the shutoff element is adjustable in the direction of the stem axis, particularly if the stopcock is of conical design, additional difficulties can arise. The conical sealing faces of the stopcock interact with a bush which is appropriately pressed into the housing and which is normally composed of polytetrafluoro-ethylene (PTFE) or a comparable material. During assembly, the stopcock has to be adjusted in the direction of the stem axis, in order to assure a sealing contact. In previously known constructions, during this adjustment, stress was also exerted on the first sealing element of the stem, and in practice an excessive pressing of the first sealing element against the stem was often found. This resulted in excessive wear of the first sealing element, with the result of an appreciably reduced lifetime and a diminished sealing effect.